Study of heterogeneous catalysis by iron-squarate based 3d metal organic framework for the transformation of tetrazines to oxadiazole derivatives

Article abstract of DOI:10.1021/ic5003258

We present here a simple, milder, and environmentally benign heterogeneous catalytic method for the transformation of tetrazines to oxadiazole derivatives at room temperature (25 °C) using our earlier synthesized iron-squarate based 3D metal organic framework, [Fe₃(OH)₃(C ₄O₄)(C₄O₄)_0.5] _n (FeSq-MOF).

Full text of DOI:10.1021/ic5003258

Communication
pubs.acs.org/IC
Study of Heterogeneous Catalysis by Iron-Squarate based 3D Metal
Organic Framework for the Transformation of Tetrazines to
Oxadiazole derivatives
Soumyabrata Goswami, Himanshu Sekhar Jena, and Sanjit Konar*
Department of Chemistry, IISER Bhopal, Bhopal-462066, India
S
* Supporting Information
regard, a challenging synthetic method using an easily
synthesizable and reusable heterogeneous catalyst would be
highly desirable.
In this work, we have explored the catalytic performance of
our earlier synthesized iron-squarate based 3D MOF,
[Fe3(OH)3(C4O4)(C4O4)0.5]n (FeSq-MOF),10 as a heteroge-
neous catalyst for the transformation of tetrazines (2a−7a) to
oxadiazole derivatives (2b−7b; Figure 1), where 2a = [3,6-
ABSTRACT: We present here a simple, milder, and
environmentally benign heterogeneous catalytic method
for the transformation of tetrazines to oxadiazole
derivatives at room temperature (25 °C) using our earlier
synthesized iron-squarate based 3D metal organic frame-
work, [Fe3(OH)3(C4O4)(C4O4)0.5]n (FeSq-MOF).
etal Organic frameworks (MOFs) or Porous Coordina-
M
tion Polymers (PCPs) are crystalline porous materials
whose structures are defined by metal ions or clusters
containing metals acting as lattice nodes and rigid bi- or
multipodal organic linkers as spacers.1,2 MOFs have many
similarities with zeolites and related microporous solids from
the structural point of view. Considering the fact that a large
number of reactions in heterogeneous catalysis are based on
zeolites and porous solids, there is interest in exploiting the
potential of MOFs as heterogeneous catalysts.3 MOFs
constitute of large density of active metal sites with free or
exchangeable coordination positions and show poor solubility
in common solvents as well as tunable and uniform pore sizes,
which make them smart heterogeneous catalysts. Thus, in a
certain way, MOFs can complement zeolites as heterogeneous
catalysts for the liquid-phase reactions. The main drawback of
MOFs is that they mostly act as Lewis acid type catalysts, and
for that the presence of coordination unsaturation at the metal
site is essential. Unfortunately, in most of the structures, the
nodal metal ions or clusters do not contain free coordination
positions to interact with substrates and reagents.4
Figure 1. Schematic representation of the transformation of tetrazines
(2a−7a) to oxadiazoles derivatives (2b−7b) at 25 °C using FeSq-
MOF.
bis(pyridin-2-yl)-1,2,4,5-tetrazine], 3a = [3,6-bis(pyridin-3-yl)-
1,2,4,5-tetrazine], 4a = [3,6-bis(pyridin-4-yl)-1,2,4,5-tetrazine],
5a = [3,6-diphenyl-1,2,4,5-tetrazine], 6a = [3,6-di-p-tolyl-
1,2,4,5-tetrazine], 7a = [4,4′-(1,2,4,5-tetrazine-3,6-diyl)-
diphenol], 2b = [2,5-bis(2-pyridyl)-1,3,4-oxadiazole], 3b =
[2,5-bis(3-pyridyl)-1,3,4-oxadiazole], 4b = [2,5-bis(4-pyridyl)-
1,3,4-oxadiazole], 5b = [2,5-bis(phenyl)-1,3,4-oxadiazole], 6b =
[2,5-bis(4-methylphenyl)-1,3,4-oxadiazole], and 7b = [2,5-
bis(4-hydroxyphenyl)-1,3,4-oxadiazole] (Table 1).
The synthesis, structural description, and magnetic properties
of the FeSq-MOF have been reported by us previously.10 It has
been found that the 3D FeSq-MOF contains two types of voids.
The larger ones are composed of aromatic squarate ligands and
hence are hydrophobic in nature. On the other hand, the
smaller ones are hydrophilic as they have coordinated hydroxyl
groups on the surface of the voids. The morphology of FeSq-
MOF has also been characterized by SEM technique. The SEM
micrograph showed well-shaped, high quality cubic crystals with
Synthesis of biologically important molecules by using MOFs
as a catalyst is a very recent and challenging attempt.5 Out of
many molecules, substituted oxadiazoles are biologically
important pharmacophores6 because of their analgesic, anti-
inflammatory, muscle relaxant, etc. activities.6d For this reason,
a large number of marketed medicines bear a 1,3,4-oxadiazole
core.6e In addition, pyridyl substituted oxadiazole derivatives
are of great research interest because of their different
coordination features and magnetic properties.7
Therefore, many synthetic strategies have been established
over the years8 to obtain oxadiazole derivatives. Among them,
the syntheses of oxadiazoles from tetrazines using peracids and
potassium hydroxide are very well-known.9 However, most of
the methods suffer a major drawback from the perspective of
environmental apprehensions like toxicity, harsh reaction
conditions, longer reaction time, cost effectiveness, difficulty
in separation, recovery, and disposal of spent catalysts. In this
Received: February 11, 2014
Published: June 30, 2014
© 2014 American Chemical Society
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Inorganic Chemistry
Communication
resulted in solid products. The ESI-MS spectra of the solids
Table 1. FeSq-MOF Mediated Transformation of 2a−7a to
2b−7b
reveal the presence of the products 2b−7b (Figures S4−S9).
1
Further, the solids were used for the H as well as 13C NMR
FeSq-MOF (5 mol %)
analysis, which matches well with the reported σ values in ppm
a
a
b
(Figures S10−S21).6a,8b
Sl. no.
S
P
T (h)
% yield
Analyzing the diameter of the hydroxyl rich pores (4.7 Å) of
the FeSq-MOF and the length of the reactant (∼11−15 Å), it
can be speculated that the above heterogeneous catalytic
transformation is occurring only on the surface of the smaller
hydroxyl rich voids. However, understanding the exact
mechanism of such a transformation is a difficult task. It can
be proposed that the tetrazine is first attacked by the OH group
coordinated to the iron center to form species I, which is
transformed to the species II via H-shift (Scheme 1).
Thereafter, ring closure occurs with the formation of the
Fe(II)-coordinated 2,3-dihydro-oxadiazolium cation (III).
In a water medium, due to the attack of the OH− ion at the
Fe(II) center, a highly unstable 2,3-dihydro-oxadiazole (IV) is
formed which subsequently oxidizes in the air to the aromatic
oxadiazole derivative (V).
1
2
3
4
5
6
2a
3a
4a
5a
6a
7a
2b
3b
4b
5b
6b
7b
1
2
97
95
96
64
55
53
1.5
24
18
20
a
b
S = Substrate; P = Product. Isolated yields of the products.
crystal sizes ranging between approximately 200 and 250 μm
(Figure S1).
An earlier report for the transformation of oxadiazoles from
tetrazines using OH− ions9a has given us a hint that the
hydroxyl richness of the framework probably makes the FeSq-
MOF a potential heterogeneous catalyst. In order to observe
the catalytic efficiency of the FeSq-MOF as a heterogeneous
catalyst, substrate 2a was allowed to stir in the presence of the
FeSq-MOF in CH3CN and H2O mixture (1:1) at 25 °C. After
stirring the heterogeneous reaction mixture for 1 h, the
conversion of 2a to 2b was noticed. Later, the same reaction
was extended for other substrates, 3a−7a. The results are
summarized in Table 1. The transformations of 2b−4b from
2a−4a were completed within 1−2 h at 25 °C in excellent
yields, whereas transformation of 5b −7b from 5a−7a showed
moderate yields and needed ∼24 h for completion at 25 °C.
The above catalytic transformations were performed in a wide
range of solvents, including a mixture of solvents, but the best
results (optimum time and significant yield) were obtained in
only a CH3CN-H2O (1:1) mixture. CH3CN was used for
dissolving the reactant/substrate, and H2O probably assisted
the catalytic transformation (Scheme 1). Although other polar
In order to find whether Fe(II) is leaching out or not, after
30 min of the reaction, FeSq-MOF was filtered off through a
Gooch-type sintered glass crucible (pore size = 15−90 μm) and
the reaction was allowed to further stir for 1.5 h. As illustrated
in Figure 2a, there was no substantial increase in yield after
Figure 2. (a) Leaching test indicating no contribution from
homogeneous catalysis of active species (A) in the presence of
FeSq-MOF, (B) after 30 min of catalyst filtration, (C) readdition of
catalyst (after 45 min). (b) Kinetic profiles in four consecutive reaction
cycles for conversion of 2b from 2a using FeSq-MOF as a catalyst.
Scheme 1. Proposed Mechanism for Transformation of
Tetrazines (2a−7a) to Oxadiazole (2b−7b) Derivatives
filtering off the catalyst. Again, after 45 min, FeSq-MOF was
added to the reaction mixture, and it was found that there is a
sudden increase in the yield of product. In overall, the stepped
reaction profile in Figure 2a showed that, on removing the
FeSq-MOF, the reaction practically stops, and re-adding the
filtered FeSq-MOF to the reaction mixture restarts it. This
simple test showed that the catalytic behavior of FeSq-MOF is
heterogeneous in nature. To draw a definitive conclusion that
iron is not leaching from the MOF, Inductively Coupled
Plasma Optical Emission Spectrometric (ICP-OES) analysis
was performed, which revealed that no metal leaching occurred
in solution.
Finally, for a more comprehensive study of the catalytic
activity of FeSq-MOF in the above reactions, a recycling test
with three consecutive runs was performed (Figure 2b). As
mentioned before, in the first cycle, 97% yield of the
transformed product (2b) was achieved after 1 h of reaction
(yield was determined from NMR analysis). Similarly, for the
other two transformed oxadiazoles (3b and 4b), yields up to
95% and 96% were achieved after 2 and 1.5 h, respectively
(Figure S22a). However, for the other transformed oxadiazoles
(5b−7b), yields of 64%, 55%, and 53% were achieved after 24,
solvents like MeOH, EtOH, etc. could also dissolve the
substrates, a greater amount of solvent was needed. Addition-
ally, it has been tested that all of the above catalytic reactions
display excellent to moderate yields using 5 mol % of the
catalyst. To ascertain the role of FeSq-MOF as a heterogeneous
catalyst, the MOF was filtered off after the completion of
reactions, and the filtrates were collected. The filtrates were
evaporated to dryness and kept at 4 °C overnight, which
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Inorganic Chemistry
Communication
Chem. 2009, 1, 695. (c) Goswami, S.; Sanda, S.; Konar, S. Cryst. Eng.
Comm. 2014, 16, 369.
18, and 20 h, respectively. For the transformation of 2b, in
second and third runs, no substantial changes in the yields
(94% and 90%) were observed for the same reaction time.
However, after three runs, an appreciable fall in the yield of 2b
has been observed down to 56% (Figure 2b). In order to reveal
any structural changes in the catalyst FeSq-MOF after four
cycles of reactions, FTIR and powder X-ray diffraction (PXRD)
analysis of recovered FeSq-MOF were performed after each
cycle of reaction, and the results are discussed in the
Supporting Information (Figures S2, S3).
To check whether Fe(II) salts are also able to convert
tetrazines to oxadiazole derivatives, the substrate tetrazines
(2a−4a) were allowed to react with Fe(II) salts (synthesis
details in Supporting Information and Table S1). Solution state
ESI-MS analysis reveals the formation of [Fe(2b/3b/
4b)]2+complexes (1−9) along with the respective transformed
products (2b−4b; Figures S26−S28). However, the reaction of
substrates (5a−7a) with Fe(II) salt indicates the stability of
only transformed products (5b−7b) in solution, which is
confirmed by ESI-MS analysis (Figure S29). Besides, complex 1
was structurally (Table S2) and magnetically characterized and
described in detail in the Supporting Information (Figure S30−
S33).
(2) (a) Bloch, E. D.; Britt, D.; Lee, C.; Doonan, C. J.; Uribe-Romo, F.
J.; Furukawa, H.; Long, J. R.; Yaghi, O. M. J. Am. Chem. Soc. 2010, 132,
14382. (b) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S.
T.; Hupp, J. T. Chem. Soc. Rev. 2009, 38, 1450. (c) Li, H.; Eddaoudi,
M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276.
(3) (a) Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H. Chem.
Commun. 2012, 48, 11275. (b) Horcajada, P.; Surble,
Hong, D.-Y.; Seo, Y.-K.; Chang, J.-S.; Greneche, J.-M.; Margiolakid, I.;
Ferey, G. Chem. Commun. 2007, 2820. (c) Lalonde, M. B.; Farha, O.
K.; Scheidt, K. A.; Hupp, J. T. ACS Catal. 2012, 2, 1550. (d) Horike,
S.; Dinca, M.; Tamaki, K.; Long, J. R. J. Am. Chem. Soc. 2008, 130,
́
S.; Serre, C.;
̀
́
̆
5854. (e) Wang, J.-L.; Wang, C.; Lin, W. ACS Catal. 2012, 2, 2630.
(f) Gascon, J.; Corma, A.; Kapteijn, F.; Xamena, F. X. L. I. ACS Catal.
2014, 4, 361. (g) Phan, N. T. S.; Nguyen, T. T.; Vu, P. H. L.
ChemCatChem. 2013, 5, 3068. (h) Gu, Z.-Y.; Park, J.; Raiff, A.; Wei, Z.;
Zhou, H.-C. ChemCatChem. 2014, 6, 67.
(4) Zou, R.-Q.; Sakurai, H.; Han, S.; Zhong, R.-Q.; Xu, Q. J. Am.
Chem. Soc. 2007, 129, 8402.
(5) Dhakshinamoorthy, A.; Garcia, H. Chem. Soc. Rev. DOI: 10.1039/
C3CS60442J.
(6) (a) Singh, S.; Sharma, L. K.; Saraswat, A.; Siddiqui, I. R.; Kehrib,
H. K.; Pal Singh, R. K. RSC Adv. 2013, 3, 4237. (b) Dolman, S. J.;
Gosselin, F.; O’Shea, P. D.; Davies, I. W. J. Org. Chem. 2006, 71, 9548.
(c) Ducharme, Y.; Blouin, M.; Brideau, C.; Chateauneuf, A.; Gareau,
Y.; Grimm, E. L.; Juteau, H.; Laliberte, S.; MacKay, B.; Masse, F.;
Ouellet, M.; Salem, M.; Styhler, A.; Friesen, R. W. ACS Med. Chem.
Lett. 2010, 1, 170. (d) Zou, X. J.; Lai, L. H.; Jin, G. Y.; Zhang, Z. X. J.
Agric. Food Chem. 2002, 50, 3757. (e) Theime, P. C.; Franke, A.;
Denke, D.; Lehmann, H. D.; Gries, J. Ger. Offen. 1981, 300, 6351.
(f) Kumar, K. A.; Jayaroopa, P.; Kumar, G. V. Int. J. ChemTech Res.
2012, 4, 1782. (g) Sahu, V. K. R.; Singh, A. K.; Yadav, D. Int. J.
ChemTech Res. 2011, 3, 1362.
(7) (a) Du, M.; Bu, X.-H.; Huang, Z.; Chen, S.-T.; Guo, Y.-M.; Diaz,
C.; Ribas, J. Inorg. Chem. 2003, 42, 552. (b) Du, M.; Guo, Y.-M.; Chen,
S.-T.; Bu, X.-H.; Batten, S. R.; Ribas, J.; Kitagawa, S. Inorg. Chem. 2004,
43, 1287.
(8) (a) Jakopin, Z.; Dolenc, M. S. Curr. Org. Chem. 2008, 12, 850.
(b) Bentiss, F.; Lagrenee, M. J. Heterocycl. Chem. 1999, 36, 1029.
́
(c) Fang, T.; Tan, Q.; Ding, Z.; Liu, B.; Xu, B. Org. Lett. 2014, 16,
2342. (d) Kidwai, M.; Bhatnagar, D.; Mishra, N. K. Green Chem. Lett.
Rev. 2010, 3, 55. (e) de Oliveira, C. S.; Lira, B. F.; Barbosa-Filho, J. M.;
Lorenzo, J. G. F.; de Athayde-Filho, P. F. Molecules 2012, 17, 10192.
(9) (a) Hunter, D.; Neilson, D. G. J. Chem. Soc., Perkin Trans. I 1985,
1081. (b) Allegretti, J.; Hancock, J.; Knutson, R. S. J. Org. Chem. 1962,
27, 1463. (c) Hoogenboom, R.; Moore, B. C.; Schubert, U. S. J. Org.
Chem. 2006, 71, 4903.
In conclusion, we present here a simple and efficient
synthetic method for the transformation of tetrazines to
oxadiazole derivatives using FeSq-MOF as a heterogeneous
catalyst at 25 °C. Compared to Fe(II) salts, FeSq-MOF exhibits
superiority in terms of product isolation, reusability, and easy-
handling. Therefore, besides being simple, milder, and
environmentally benign, this catalytic method gives easy access
to the synthesis of the chemically and biologically important
oxadiazole derivatives.
ASSOCIATED CONTENT
■
S
* Supporting Information
Includes experimental details, a scheme, ESI-MS, H and 13C
1
NMR, PXRD, structural and magnetic study detail, and
crystallographic tables. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Tel: +91-755-6692339. Fax: +91-755-6692392. E-mail:
skonar@iiserb.ac.in.
■
(10) Goswami, S.; Adhikary, A.; Jena, H. S.; Biswas, S.; Konar, S.
Inorg. Chem. 2013, 52, 12064.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.G. thanks IISER Bhopal for a Ph.D. fellowship. H.S.J. thanks
IISER Bhopal for a postdoctoral fellowship. The authors
sincerely thank Mr. Qysar Maqbool for his help in SEM
measurement. S.K. thanks CSIR, Government of India (Project
No. 01(2473)/11/EMR-II) and IISER Bhopal for generous
financial and infrastructural support.
DEDICATION
■
Dedicated to Professor Abraham Clearfield on the occasion of
his 86th birthday.
REFERENCES
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(1) (a) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed.
2004, 43, 2334. (b) Horike, S.; Shimomura, S.; Kitagawa, S. Nat.
7073
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